Phase shifting interferometry is a well-known (Kreath) technique by which surface contour maps may be generated. Various optical techniques may be used to produce a contour or "fringe" map of the surface to be quantified. Coherent interferometers and moire interferometers are among the most common types of full field surface contouring devices. Phase-shifted interferometry increases the resolution of the depth measurement of the raw fringe contour map, as well as sense of slope. Phase shifting is traditionally implemented by digitizing multiple images of a sample object with a CCD video camera. Each image is captured with a slight change in the phase of the interferometric contour map. Phase shift analysis is applied to the images. This phase shift analysis computes a relative contour map of the object surface. A common phase shift analysis is referred to as a "bucket" algorithm, where each phase image is referred to as a "bucket". A minimum number of 3 buckets is typically required for the bucket algorithm.
Between each bucket image, translating the position of one or more optical components traditionally changes the phase. This translation may occur specifically between each image capture or "on the fly" with a precise velocity control timed to the data rate of the CCD camera. In the case of a moire interferometer, the translated object is usually one of the gratings although other components may be translated. (See, U.S. Pat. Nos. 5,069,548 and 5,646,733, described in greater detail hereinbelow). Coherent interferometers manipulate the reference optical path length by moving a reference mirror or window.
It is critical to the success of the phase shift that the object and the optical system remain completely stationary with respect to each other. Vibrations and/or motion of the object will degrade the quality of the data. In some cases, such motion can actually cause the system to fail.
In many instances, particularly in process inspection of manufactured products, the precision surface contour of phase shifting interferometry may be desirable. However, the length of time required to produce a number of bucket images while the object is held stationary relative to the optical system precludes its use.
In addition, variations in the phase shifting mechanism, such as backlash, part wear, and drifts in the timing sequence, will cause non-linearity and changes in the accuracy of the machine, which degrade the repeatability of the measurement.
U.S. Pat. No. 5,880,844 discloses a method to detect the height of features in an image by the difference in the degree of focus at each of the detectors, which are aligned such that their planes of best focus are at different locations in the object space.
U.S. Pat. No. 5,793,900 discloses a method identical to U.S. Pat. No. 5,880,844. Differences include the image processing and field of view size. U.S. Pat. No. 5,793,900 seeks to map topography on a scale suitable for robot guidance while U.S. Pat. No. 5,880,844 is a microscopy technique. However, the fundamental principle remains height mapping based on the quality of focus.
U.S. Pat. No. 4,942,618 discloses an optical configuration substantially identical to those disclosed in the patents noted immediately above. In this patent, the image processing is based on contrast and size of objects in the image rather than on sharpness of the objects. The image processing is based on the assumption that the object being measured is known to be a wire or like object.
U.S. Pat. No. 5,852,672 discloses a technique that is based on stereo imaging. This method requires that each of at least two cameras are positioned such that they are observing the same object space, but from distinct points of view. Projected line or grating patterns are added as an enhancement to the stereo technique.
U.S. Pat. No. 5,774,221 discloses the use of phase-controlled evanescent illumination. The patent discusses methods for controlling or manipulating the phase of inhomogeneous waves. The patent further discusses methods for applying this technology to the measurement of surface microtopography. Two detectors are positioned at two specific locations. One system detects diffuse type reflections, the other collects specular type reflections. Further, the principle utilized is based on detecting the phase of the illumination light waves.
U.S. Pat. No. 5,416,591 is a scanning laser technique that references an array sensor, and three-dimensional surface mapping. The method is common to flying spot laser scanners. It is unique in that it has the capability of scanning in two dimensions, hence the reference to the array sensor (basically a CCD array), rather than a line sensor. The method actually uses only one plane of focus for the detector(s); each individual element of the array is sensing a separate location in the image plane.
U.S. Pat. No. 5,069,548 describes a method of translating the entire grating projection system as a means of obtaining absolute depth measurements as opposed to the traditional relative surface measure. The system does not employ multiple cameras, and requires moving parts of the system to obtain its data.
U.S. Pat. No. 5,307,152 discloses two cameras which view an object area from two separate positions. The cameras are completely independent of one another. This method relies on traditional phase-shifted moire techniques including physical translation of the grating.
U.S. Pat. No. 5,646,733 discloses a method and system including an optical head which moves relative to an object at a vision station to scan a projected pattern of imagable electromagnetic radiation across the surface of an object to be inspected at a relatively constant linear rate to generate an imagable electromagnetic radiation signal.
U.S. Pat. No. 5,608,529 discloses a system which projects a pattern onto the object being observed. The projection is done in such a way that the pattern is in focus at three discrete depth locations in the object space. The three patterns are observed with three separate detectors, each aligned such that they have discrete planes of observation in the object space. This patent discloses depth measurement by common triangulation techniques, while extending the depth range by observing the quality of the focus at discrete image locations.
SPIE Vol. 3204 includes a paper entitled "3D Imaging Using A Unique Refractive Optic Design To Combine Moire and Stereo" authored by L. Bieman and K. Harding. The technique described therein has a limited inspection area which is equal to or less than the diameter of the main imaging lens. In addition, resolution is limited to 1/3 the detector area.